As portable electronic devices become more intertwined with everyday life of people, it becomes necessary to put more functionality into these devices, run them at higher circuit speeds, and have them consume smaller amounts of energy. These electronic devices are becoming smaller and lighter and are often required to operate with Liquid Crystal Displays (LCDs) for increasing periods of time. Unfortunately, the battery capacities are increasing at a much slower pace than the overall power dissipation of this class of electronic devices. Therefore, it is essential to develop design techniques to reduce the overall power dissipation of these devices.
In many of these devices, the energy consumption in the Cold Cathode Fluorescent Lamp (CCFL), which is the backlight of the LCD, dominates the overall energy consumption of the device. In some cases, the display backlight accounts for almost 50% of the battery drain when the display is at maximum intensity.
FIG. 1 shows the typical architecture of the digital LCD subsystem 100 in a microelectronic device. There are two main components in this subsystem: a) the graphics controller 110, which includes the video controller 111 and frame buffer memory 112 and b) the LCD component 120, which includes the LCD controller 121 and LCD panel 122. The image data, which is received from the processing unit, is first saved into the frame buffer memory 112 by the video controller 111 and is subsequently transmitted to the LCD controller 121 through an appropriate analog (e.g., VGA) or digital (e.g., DVI) interface 130. The LCD controller 121 receives the video data and generates a proper grayscale (i.e., transmissivity of the panel 122) for each pixel based on its pixel value. All of the pixels on a transmissive LCD panel are illuminated from behind by the backlight. To the observer, a displayed pixel looks bright if its transmittance is high (i.e., it is in the ‘on’ state), meaning it passes the backlight. On the other hand, a displayed pixel looks dark if its transmittance is low (i.e., it is in the ‘off’ state), meaning that it blocks the backlight. For color LCDs, different filters are used to generate shades of three main colors (i.e. red, blue, and green), and then color pixels are generated by mixing three sub-pixels together to produce different colors.
FIG. 2 depicts the LCD component 200 in more detail. The data received from the video bus 130 is used to infer timing information and respective grayscale levels for a row of pixels. Next, the pixel values are converted to the corresponding voltage levels to drive the thinfilm-transistors (TFT.s) on different columns of the selected row. The backlight bulb 221 is powered with the aid of a DC-AC converter 222, to provide the required illumination of the LCD matrix 223.
FIG. 3 illustrates a schematic for a common TFT cell 300. Each pixel has an individual liquid crystal cell, a TFT 310, and a storage capacitor. The electrical field of the capacitor controls the transmittance of the liquid crystal cell. The capacitor is charged and discharged by the TFT. The gate electrode of the TFT controls the timing for charging/discharging of the capacitor when the pixel is scanned (or addressed) by the tracer for refreshing its content. The (drain-) source electrode of the TFT controls the amount of charge. The gate electrodes and source electrodes of all TFTs are driven by a set of gate drivers and source drivers, respectively. A single gate driver (called a gate bus line 320) drives all gate electrodes of the pixels on the same row. The gate electrodes are enabled at the same time the row is traced. A single source driver (called a source bus line 330) drives all source electrodes of the pixels on the same column. The source driver 330 supplies the desired voltage level (called grayscale voltage) according to the pixel value. In other words, ideally, the pixel value transmittance, t(X), is a linear function of the grayscale voltage v(X), which is in turn a linear function of the pixel value X. The transfer function of source driver 330, which maps different pixel values, X, into different voltage levels, v(X) is called the grayscale-voltage function. If there are 256 grayscales, then the source driver 330 must be able to supply 256 different grayscale voltage levels. For the source driver 330 to provide a wide range of grayscales, a number of reference voltages are required. The source driver 330 mixes different reference voltages to obtain the desired grayscale voltages. Typically, these different reference voltages are fixed and designed as a voltage divider. Mathematically speaking, in a transmissive TFT-LCD monitor, for a pixel with value X, the luminance I(A) of the pixel is: I(X)=b.t(X) where t(A) is the transmissivity of the TFT-LCD cell for pixel value X, and bε[0,1] is the (normalized) backlight illumination factor with b=1 representing the maximum backlight illumination and b=0 representing no backlight. It should be appreciated that t(X) is a linear mapping from [0,255] domain to [0,1] range. In backlight scaled TFT-LCD, b is scaled down and accordingly t(X) is increased to achieve the same image luminance.
Previous approaches cannot fully utilize the power saving potential of the dynamic backlight scaling scheme because their measure of distortion between the original and the backlight-scaled image is an overestimation. This is because these approaches simply either minimize the number of saturated pixel values or maximize the number of pixel values that are preserved. Image distortion (more precisely, the difference between a pair of similar images) is a complex function of the visual perception, and hence, it cannot be correctly evaluated by comparing the images pixel by pixel (i.e., calculating the root mean squared error of the corresponding pixel values) or as a whole (i.e., using the integral of the absolute value of the histogram differences). A correct measure of distortion should appropriately combine the mathematical difference between pixel values (or histograms) and the characteristics of the human visual system.